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Insert imagehere

ABB s.r.o.VT Guard

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Damping resistor

Parallel conection

Lower resistance means betterdamping of resonanceoscillation.

Higher resistance - to providepower safety, when one phase isgrounded.

ResistanceR – 22 Ohm PR-500W

R – 27 Ohm PR-500W

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Damping resistor - Conflict

Ferroresonace appears =>low resistance needed

Phase grounded =>high resistance needed

Damping resistor – only a particular solution

N

A

n

a

dn

da

N

A

n

a

dn

da

N

A

n

a

dn

da R1

R

S

T

VT1 VT2 VT3

U0

FDC

R2

1

2

R

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Solution is called - VT guard

For damping low resistance cca. 15 OhmFor one phase grounded resistance high >> 200 Ohm

VT Guard - conneted in open delta (with VT’s residual windings)

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VT guard basic function

Fig. 1. Oscillating voltage and currentdiagram when ferroresonance appears.

Fig. 2. Experimental demonstrationof the VT guard effectiveness.

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VT guard

Size according DIN – You can put it inlow voltage side in switchgear.

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VT Guard installation

VT Guard

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Comparison: Resistor vs. VT Guard

No significant temperature increaseHigh temperature increase

Reliable ferroresonance security deviceNot reliable (depending onnetwork conditions)

Complex solution

Phase grounded – high resistanceFerroresonace appears – low resistance

Particular solution

Fast dampingSlow damping

Mounted in the low-voltage part on DIN-railMounted in the near of the voltagetransformers in the primary part

VT GuardResistor

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VT Guard : features and benefits

Effective ferroresonance dampingdue to low resistance

No thermal hazard to VT (realthermal protection)

Nonsensitive to natural networkasymmetry

Small size (DIN –rail mounted)

Applicable to any type of voltagetransformers equipped with residualwindings

New solution – no competitor

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